With recent advances in DNA sequencing technology, profiling one's own genetic information is becoming a reality for many patients and their families. In fact, assaying for mutated genes known to increase the risk of breast cancer has become commonplace for individuals at high risk. Hereditary forms of breast cancer comprise approximately 20,000 cases annually in the United States. It is believed that the breast and ovarian cancer susceptibility genes, BRCA1 and BRCA2, account for the majority of germ-line mutations in families with a genetic predisposition for developing breast and/or ovarian cancers. Since their discovery in the mid 1990's, over a thousand different alleles have been associated with each gene and these DNA germline mutations have been catalogued in the Breast Cancer Database. Unfortunately, a large number of germline changes reported for either BRCA1 or BRCA2 are not definitively linked to disease. These genomic alterations have been termed variants of uncertain significance (VUS) and their role in breast carcinogenesis remains unclear. In addition, penetrance of known deleterious mutations varies widely among carriers and it is hypothesized this is due to both genetic and environmental modifiers. This variability creates important clinical challenges in assessing carrier risk, since many individuals elect for prophylactic procedures or chemopreventive measures that are expensive and associated with significant side effects. Using modern somatic cell gene targeting techniques, we are developing a novel model to assess relative risk for deleterious BRCA alleles. In addition, we will use this model to further categorize VUS as either benign or deleterious. By thoroughly characterizing BRCA alleles, we hope to provide additional knowledge for further risk stratification that could be incorporated with many current models. This will aid clinicians and families that harbor these mutations/VUS in making decisions when considering the risks and benefits of prophylactic surgeries and other primary prevention strategies.